linux-kernel - Re: [PATCH v10 2/5] sched: CGroup tagging interface for core scheduling

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Message-ID: <YBv8gBSKU1MdtXuW@hirez.programming.kicks-ass.net>
Date:   Thu, 4 Feb 2021 14:54:08 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     Nishanth Aravamudan <naravamudan@...italocean.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Vineeth Pillai <viremana@...ux.microsoft.com>,
        Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.intel@...il.com>, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, mingo@...nel.org,
        torvalds@...ux-foundation.org, fweisbec@...il.com,
        keescook@...omium.org, kerrnel@...gle.com,
        Phil Auld <pauld@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>, vineeth@...byteword.org,
        Chen Yu <yu.c.chen@...el.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Agata Gruza <agata.gruza@...el.com>,
        Antonio Gomez Iglesias <antonio.gomez.iglesias@...el.com>,
        graf@...zon.com, konrad.wilk@...cle.com, dfaggioli@...e.com,
        pjt@...gle.com, rostedt@...dmis.org, derkling@...gle.com,
        benbjiang@...cent.com,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        James.Bottomley@...senpartnership.com, OWeisse@...ch.edu,
        Dhaval Giani <dhaval.giani@...cle.com>,
        Junaid Shahid <junaids@...gle.com>, jsbarnes@...gle.com,
        chris.hyser@...cle.com, Ben Segall <bsegall@...gle.com>,
        Josh Don <joshdon@...gle.com>, Hao Luo <haoluo@...gle.com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH v10 2/5] sched: CGroup tagging interface for core
 scheduling

On Fri, Jan 22, 2021 at 08:17:01PM -0500, Joel Fernandes (Google) wrote:
> From: Peter Zijlstra <peterz@...radead.org>

I'm thinking this is too much credit, I didn't write much of this.

> Marks all tasks in a cgroup as matching for core-scheduling.
> 
> A task will need to be moved into the core scheduler queue when the cgroup
> it belongs to is tagged to run with core scheduling.  Similarly the task
> will need to be moved out of the core scheduler queue when the cgroup
> is untagged.
> 
> Also after we forked a task, its core scheduler queue's presence will
> need to be updated according to its new cgroup's status.
> 
> Use stop machine mechanism to update all tasks in a cgroup to prevent a
> new task from sneaking into the cgroup, and missed out from the update
> while we iterates through all the tasks in the cgroup.  A more complicated
> scheme could probably avoid the stop machine.  Such scheme will also
> need to resovle inconsistency between a task's cgroup core scheduling
> tag and residency in core scheduler queue.
> 
> We are opting for the simple stop machine mechanism for now that avoids
> such complications.
> 
> Core scheduler has extra overhead.  Enable it only for core with
> more than one SMT hardware threads.

Very little actual words on the desired and implemented semantics of the
interface, while the patch contains much non-obvious complication.